Survival depends on being able to accurately determine if an environmental stimulus requires a behavioral response. For example, the nervous system must set an appropriate behavioral threshold that an auditory stimulus must surpass in order to trigger a startle response. This threshold should be set such that potentially dangerous situations are detected and averted yet not so low that common sounds will elicit a response. Excessive startle responses are observed in many neuropsychiatric disorders including schizophrenia, post-traumatic stress disorder, anxiety disorders, and addiction. Furthermore, this threshold should be able to be modulated so that, for instance, an individual will become habituated to persistent loud stimuli. While the principle components of the hindbrain and spinal cord circuits that underlie the startle response are known and conserved among vertebrates, the molecular-genetic mechanisms that regulate startle threshold and modulation are not well understood. Here I propose to use the powerful zebrafish model system to investigate the mechanisms of startle regulation. In fish the acoustic startle response is initiated by the firing of one of two bilateral giant reticulospinal neurons, the Mauthner cells, which receive direct synaptic input from the ipsilateral auditory nerve. Upon firing, the Mauthner cell directly activates contralateral motor neurons to trigger a characteristic C-bend, initiating escape behavior. Through a recent genetic screen for mutants with defects in startle modulation we identified a mutant that is hypersensitive to acoustic startle stimuli. Without displaying hyperactivity or any other defects in startle kinematics, homozygous houdini larvae perform startle escape responses to low-level auditory stimuli that fail to elicit escapes in wild-type fish. This suggests that the houdini gene plays a role in setting the acoustic startle threshold. The aims in this proposal will determine the extent of houdini's role in regulating behavior and identify the mechanisms by which it modulates the acoustic startle threshold. In aim 1 I will analyze whether houdini larvae are also hypersensitive to stimuli in other sensory modalities and whether they show defects in startle modulation such as habituation and prepulse inhibition. I will also test houdini adults for hypersensitivity and in anxiety, aggression, and addiction paradigms. In aim 2 I will identify the houdini gene and characterize its spatiotemporal expression. And in aim 3 I will reveal molecular mechanisms by which houdini affects the excitability of the Mauthner cell startle circuit. The houdini mutant presents an exciting opportunity to understand, at least in part, how the nervous system decides whether to initiate a behavior. These experiments will open up avenues not only for further characterization of the startle pathway but also for potential therapeutic interventions in conditions such as anxiety disorders.